Hybrid electric vehicles (HEVs) and electric vehicles (EVs), for example, each commonly incorporate an assembled battery (battery system) that includes a plurality of single battery cells connected in series with each other in a secondary battery in order to achieve a desired high voltage. Such a traditional assembled battery includes a battery monitoring circuit that incorporates, for example, an integrated circuit connected with each of a predetermined number of single battery cells. The battery monitoring circuit performs, for example, measurement of a voltage across terminals of each single battery cell (cell voltage) and balancing discharge for equalizing residual capacity of each single battery cell to thereby monitor and manage a state of each single battery cell. During balancing, each single battery cell is discharged depending on the residual capacity thereof and a discharge current flows to a balancing resistor via a voltage detection line disposed between each single battery cell and the battery monitoring circuit. At this time, a voltage drop corresponding to the magnitude of an impedance occurs in the voltage detection line.
Recent years have witnessed practical application of single battery cells that exhibit fluctuations in voltage smaller than ever before with respect to a change in the residual capacity. Use of such a single battery cell requires measurement accuracy higher than before in order to make accurate estimation of the residual capacity through measurement of the cell voltage. Thus, in the measurement of the cell voltage during balancing, an effect from the abovementioned voltage drop in the voltage detection line is not negligible. Hence, a method for accurately measuring the cell voltage has been developed by correcting the voltage drop in the voltage detection line (see PTL 1).